Last time: is a population evolving?




  • Re-examine the beetle populations
    • color: one locus, 2 alleles


  • Across 2 generations, are allele frequencies the same?


  • What does this mean?

Hardy Weinberg describes a population NOT evolving

Allele frequency change in beetles: Mutation


  • Mutations in the beetle color gene, resulting in more brown
    • to rare to be considered evolutionary change


  • Mutation rate =
    • mutation rates are generally small (~0.0001%).
    • humans = 2.5 x 10-8 mutations nucleotide-1 generation-1
    • negligible change from one generation to the next


  • Mutation may counterbalance selection
    • selection against a deleterious recessive allele (a)
    • but mutation from A -> a
    • selection may not completely eliminate the ‘a’ allele

Allele frequencies change in beetles: Genetic Drift


  • Genetic drift describes how allele frequencies fluctuate unpredictably across generations
    • happens in every population
    • pronounced in small populations
    • extremely important evolutionary “force”


  • Reduce genetic variation through the random loss of alleles


  • In a small population, beetles are green or brown
    • several green beetles get stepped on by Dr. C
    • next generation has more brown beetles, by chance
    • did it matter if alleles were dominant or recessive?

Hypothetical coin flips to choose 2 alleles

Mechanism of Genetic Drift: Founder Effect




  • Founder Effect: when a few individuals become isolated from a larger population
    • some alleles make it, others lost
    • non-random


  • Occurs frequently on islands
    • what could happen with bad alleles?

Mechanism of Genetic Drift: Bottleneck Effect



  • Bottlenecks occurs after a drastic reduction in population size
    • sudden change in the environment
    • fire, flood, invasive species, etc.


  • The new gene pool may be different from original population


  • If the population stays small, genetic drift could increase
    • black robins on Chatham island

Bottlenecks, species habitat loss, and conservation

Allele frequencies change in beetles: Gene Flow


  • Gene flow: exchange of alleles between 2 or more populations
    • aka ‘Gene migration’
    • like mutation but way more powerful


  • Occurs when new individual migrates and mates successfully
    • brown beetles enters population
    • changes allele frequency for color


  • Gene flow can be good or bad
    • may increase genetic diversity
    • bring harmful alleles


  • Does gene flow make populations more or less unique?

Allele frequencies change in beetles: Natural Selection

Only natural selection causes adaptive evolution!


  • ‘Sorting’ of alleles that favor reproductive success
    • leads to adaptation


  • Certain traits lead to greater Relative Fitness
    • individuals with good trait (alleles) will contribute more to gene pool
    • better survival/reproduction compared to others


  • More individuals in a population with greater relative fitness means….


  • Natural selection is a ‘moving target’…WHY?

Natural Selection: selection of heritable traits